The vapor phase reaction of HF with halogenated aliphatic compounds over solid catalysts is well known in the art. The use as a catalyst for these reactions of chromium (III) oxide, sometimes referred to as dichromium trioxide of Cr.sub.2 O.sub.3, is also known.
These chromium (III) oxides can be made by the reduction of chromium (VI) oxide (CrO.sub.3) with ethanol. Because of the physical form of the product obtained, this product is often referred to as "gel catalyst." The catalysts can also be made from the commercial pigment Guignet's Green, a chromium (III) oxide with the empirical formula Cr.sub.2 O.sub.3.sup.. 2 H.sub.2 O; and by precipitation of a water-containing chromium (III) trihydroxide by treating aqueous solutions of chromium (III) nitrate or other water-soluble salts with an alkaline reagent. Because the trihydroxide Cr(OH).sub.3 is empirically chromium (III) oxide trihydrate Cr.sub.2 O.sub.3.sup.. 3 H.sub.2 O, this catalyst is often referred to as "COT" catalyst. Although chromium (III) oxides so obtained have long been considered to be amorphous, recent studies have shown that the chromium (III) oxides so prepared exhibit an orthorhombic crystal structure, and are characterized as gamma-chromium oxide hydroxide, usually written as .gamma.-CrOOH.
.gamma.-CrOOH has been recognized as an excellent fluorination catalyst due to its high activity in the replacement of other halogens with fluorine. It is among the few catalysts, for example, that will replace all four chlorine atoms in CCl.sub.4 to form CF.sub.4. In addition to good activity for the desired reaction, however, a catalyst should be productive in the sense of promoting the reaction of a large quantity of reactant with a small quantity of catalyst. An especially significant commercial catalyst characteristic is longevity, or the retention of catalyst activity and productivity over an extended period of time. Replacement of catalyst not only represents a significant expense in itself, but the time spent in replacing the catalyst detracts from the productivity of a plant in which the catalyst is used.